This invention relates to a system for feedback control and air/fuel ratio in an internal combustion engine, which system includes an air/fuel ratio detector having an oxygen-sensitive element of an oxygen concentration cell type disposed in the exhaust gas, provided with an electric heater to ensure proper function of this element and operated with the supply of a DC current to establish a reference oxygen partial pressure in this element, and more particularly to a sub-system to control both the magnitude of a voltage applied to the heater and the intensity of the aforementioned current according to the operating conditions of the engine.
In recent internal combustion engines and particularly in automotive engines, it has become popular to control the air/fuel mixing ratio precisely to a predetermined optimal value by performing feedback control to thereby improve the efficiencies of the engine and reducing the emission of noxious or harmful substances contained in exhaust gases.
For example, in an automotive engine system including a catalytic converter which is provided in the exhaust passage and contains a so-called three-way catalyst that can catalyze both the reduction of nitrogen oxides and oxidation of carbon monoxide and unburned hydrocarbons, it is desirable to control the air/fuel mixing ratio to a stoichiometric ratio because this catalyst exhibits highest conversion efficiencies in an exhaust gas produced by combustion of a stoichiometric air-fuel mixture, and also because the employment of a stoichiometric mixing ratio is favorable for realization of high mechanical and thermal efficiencies of the engine. It has been put into practice to perform feedback control of air/fuel ratio in such an engine system by using a sort of oxygen sensor, which is installed in the exhaust passage upstream of the catalytic converter, as a device that provides an electrical feedback signal indicative of the air/fuel ratio of an air-fuel mixture actually supplied to the engine. Based on this feedback signal, a control circuit commands a fuel-supplying apparatus such as electronically controlled fuel injection valves to control the rate of fuel feed to the engine so as to correct deviations of actual air/fuel ratio from the intended stoichiometric ratio.
Usually the above mentioned oxygen sensor is of an oxygen concentration cell type utilizing an oxygen ion conductive solid electrolyte, such as zirconia stabilized with yttria or calcia. According to a well known design, the sensor is constituted fundamentally of a solid electrolyte layer in the shape of a tube closed at one end and two porous electrode layers formed on the outer and inner surfaces of the solid electrolyte tube, respectively. When there is a difference in oxygen partial pressure between the outer electrode side and inner electrode side of the solid electrolyte layer, this sensor generates an electromotive force between the two electrode layers. As an air/fuel ratio detector for the above-mentioned purpose, the outer electrode layer is exposed to an engine exhaust gas while the inner electrode layer is exposed to atmospheric air utilized as the source of a reference oxygen partial pressure. In this state the magnitude of the electromotive force exhibits a great and sharp change between a maximally high level and a very low level each time when the air/fuel ratio of a mixture supplied to the engine changes across the stoichiometric ratio. Accordingly it is possible to produce a fuel feed rate control signal based on the result of a comparison of the output of the oxygen sensor with a reference voltage which has been set at the middle of the high and low levels of the sensor output.
However, this type of oxygen sensor has disadvantages such as significant temperature dependence of its output characteristics, necessity of using a reference gas such as air, difficulty in reducing the size and insufficiency of mechanical strength.
To eliminate such disadvantages of the conventional oxygen sensor and enable to detect exact air/fuel ratio values for not only a stoichiometric or nearly stoichiometric mixture but also a distinctly non-stoichiometric mixture, U.S. Pat. Nos. 4,207,159 and 4,224,113 disclose an advanced device comprising an oxygen-sensitive element in which an oxygen concentration cell is constituted of a lamination of a flat and microscopically porous layer of a solid electrolyte, a measurement electrode layer porously formed on one side of the solid electrolyte layer and a reference electrode layer formed on the other side, with the provision of a substrate such that the reference electrode layer is tightly sandwiched between the substrate and the solid electrolyte layer and macroscopically shielded from the environmental atmosphere. Each of the three layers on the substrate can be formed as a thin, film-like layer. This device does not use any reference gas. Instead, a DC power supply means is connected to the oxygen-sensitive element so as to force a constant DC current (e.g. of an intensity of about 20 microamperes) to flow through the solid electrolyte layer between the two electrode layers to thereby cause migration of oxygen ions through the solid electrolyte layer in a desired direction and, as a consequence, establish a reference oxygen partial pressure at the interface between the reference electrode layer and the solid electrolyte layer, while the measurement electrode layer is allowed to contact an engine exhaust gas. Where the current is forced to flow in the solid electrolyte from the reference electrode layer towards the measurement electrode layer, there occur ionization of oxygen contained in the exhaust gas at the measurement electrode and migration of negatively charged oxygen ions through the solid electrolyte layer towards the reference electrode. The rate of supply of oxygen in the form of ions to the reference electrode is primarily determined by the intensity of the current. The oxygen ions arrived at the reference electrode layer are deprived of electrons and turn into oxygen molecules to result in accumulation of gaseous oxygen on the reference electrode side of the concentration cell. However, a portion of the accumulated oxygen molecules diffuse outwardly through the microscopical gas passages in the solid electrolyte layer. Therefore, it is possible to maintain a constant and relatively high oxygen partial pressure which serves as a reference oxygen partial pressure on the reference electrode side of the concentration cell by the employment of an appropriate current intensity with due consideration of the microscopical structure and activity of the solid electrolyte layer. Then generated between the reference and measurement electrode layers of this oxygen-sensitive element is an electromotive force of which the magnitude is related to the composition of the exhaust gas and the air/fuel ratio of a mixture from which the exhaust gas is produced. Also it is possible to operate this oxygen-sensitive element by forcing a DC current to flow therein from the measurement electrode layer towards the reference electrode layer. In this case a constant and relatively low oxygen partial pressure can be maintained at the interface between the reference electrode layer and the solid electrolyte layer.
The device according to U.S. Pat. Nos. 4,207,159 and 4,224,113 has advantages such as unnecessity of using any reference gas, excellence in sensitivity or responsiveness, ability of detecting numerical values of air/fuel ratios which may be either above or below a stoichiometric ratio, possibility of producing it into a very small size and good resistance to mechanical shocks and vibrations.
In practical applications it becomes necessary to provide this advanced oxygen-sensitive element (also conventional oxygen sensors of the solid electrolyte concentration cell type) with an electric heater because the activity of the solid electrolyte in the element becomes unsatisfactorily low while the temperature of the element is relatively low, e.g. below about 500.degree. C., so that the element installed in an engine exhaust system becomes ineffective as an air/fuel ratio detecting element while the engine discharges a relatively low temperature exhaust gas if the element should be heated solely by the heat of the exhaust gas. The electric heater is usually attached to, or embedded in, the substrate of the oxygen-sensitive element.
A problem recognized in the applications of the air/fuel ratio detector according to the above quoted U.S. Patents to feedback-type air/fuel ratio control systems for automotive engines is a fact that the magnitude of the above described reference oxygen partial pressure in the oxygen-sensitive element varies considerably under certain operating conditions of the engine even though the intensity of the DC current supplied to the concentration cell part of the element is kept constant. More exactly, the magnitude of the reference oxygen partial pressure is influenced by the temperature of the exhaust gas and the amount of oxygen contained in the exhaust gas.
When the exhaust gas is very high in temperature and considerably low in the concentration of oxygen therein as in the case of the engine being operated under a full-throttle or nearly full-throttle accelerating condition with the feed of a fuel-enriched mixture, the reference oxygen partial pressure (produced by forcing a constant DC current to flow in the solid electrolyte layer towards the measurement electrode layer) lowers greatly and becomes practically zero in an extreme case. Because, although the migration of oxygen ions through the solid electrolyte layer towards the reference electrode layer by the effect of the flow of the constant current continues, the outward diffusion of gaseous oxygen from the reference electrode through the solid electrolyte into the exhaust gas of a low oxygen concentration augments. Therefore, it becomes impossible to continue the feedback control of air/fuel ratio correctly. It is conceivable to suspend the feedback control during operation of the engine under such an extremely high-load condition, but when the control is resumed it takes a relatively long period of time for the lowered reference oxygen partial pressure to recover the initially intended magnitude compared with the frequencies of the feedback signal produced by the air/fuel ratio detector and the control signal provided to the fuel supply apparatus, so that during this time period it becomes impossible to accurately control the air/fuel ratio.
On the contrary, there occurs a great increase in the magnitude of the reference oxygen partial pressure attributed to the flow of the same DC current when the exhaust gas temperature is very low, and particularly when the oxygen concentration in the exhaust gas is considerably high as in the case of a great deceleration of the engine operation with a temporary interruption of the feed of fuel or with the feed of a very lean mixture. The reason is that under such a condition there occurs an increase in the amount of oxygen ions supplied to the reference electrode layer relative to the amount of oxygen molecules diffusing outwardly from the reference electrode through the solid electrolyte layer because of the increased oxygen concentration in the exhaust gas and lowering of the activity of the solid electrolyte by the effect of the lowered exhaust gas temperature. Correct feedback control of air/fuel ratio becomes impossible also in this case. Besides, when the reference oxygen partial pressure continues to augment by this reason beyond a certain critical level, there is a strong possibility of breakage of the oxygen-sensitive element which is constituted fundamentally of relatively thin layers.